In multicellular animals, cell growth, differentiation, and migration are controlled by polypeptide growth factors. These growth factors play a role in both normal development and pathogenesis, including the development of solid tumors.
Polypeptide growth factors influence cellular events by binding to cell-surface receptors, many of which are tyrosine kinases. Binding initiates a chain of signalling events within the cell, which ultimately results in phenotypic changes, such as cell division, protease production, and cell migration.
Growth factors can be classified into families on the basis of structural similarities. One such family, the PDGF (platelet derived growth factor) family, is characterized by a dimeric structure stabilized by disulfide bonds. This family includes PDGF, the placental growth factors (PlGFs), and the vascular endothelial growth factors (VEGFs). The individual polypeptide chains of these proteins form characteristic higher-order structures having a bow tie-like configuration about a cystine knot, formed by disulfide bonding between pairs of cysteine residues. Hydrophobic interactions between loops contribute to the dimerization of the two monomers. See, Daopin et al., Science 257:369, 1992; Lapthorn et al., Nature 369:455, 1994. Members of this family are active as both homodimers and heterodimers. See, for example, Heldin et al., EMBO J. 7:1387-1393, 1988; Cao et al., J. Biol. Chem. 271:3154-3162, 1996. The cystine knot motif and bow tie fold are also characteristic of the growth factors transforming growth factor-beta (TGF-β) and nerve growth factor (NGF), and the glycoprotein hormones. Although their amino acid sequences are quite divergent, these proteins all contain the six conserved cysteine residues of the cystine knot.
Four vascular endothelial growth factors have been identified: VEGF, also known as vascular permeability factor (Dvorak et al., Am. J. Pathol. 146:1029-1039, 1995); VEGF-B (Olofsson et al., Proc. Natl. Acad. Sci. USA 93:2567-2581, 1996; Hayward et al., WIPO Publication WO 96/27007); VEGF-C (Joukov et al., EMBO J. 15:290-298, 1996); and VEGF-D (Oliviero, WO 97/12972; Achen et al., WO 98/07832). Five VEGF polypeptides (121, 145, 165, 189, and 206 amino acids) arise from alternative splicing of the VEGF mRNA.
VEGFs stimulate the development of vasculature through a process known as angiogenesis, wherein vascular endothelial cells re-enter the cell cycle, degrade underlying basement membrane, and migrate to form new capillary sprouts. These cells then differentiate, and mature vessels are formed. This process of growth and differentiation is regulated by a balance of pro-angiogenic and anti-angiogenic factors. Angiogenesis is central to normal formation and repair of tissue, occuring in embryo development and wound healing. Angiogenesis is 0.25 also a factor in the development of certain diseases, including solid tumors, rheumatoid arthritis, diabetic retinopathy, macular degeneration, and atherosclerosis.
A number of proteins from vertebrates and invertebrates have been identified as influencing neural development. Among those molecules are members of the neuropilin family and the semaphorin/collapsin family.
Neuronal cell outgrowths, known as processes, grow away from the cell body to form synaptic connections. Long, thin processes that carry information away from the cell body are called axons, and short, thicker processes which carry information to and from the cell body are called dendrites. Axons and dendrites are collectively referred to as neurites. Neurites are extended by means of growth cones, the growing tips of neurites, which are highly motile and are ultimately responsible for increasing and extending the neuronal network in the body.
Three receptors for VEGF have been identified: KDR/Flk-1 (Matthews et al., Proc. Natl. Acad. Sci. USA 88:9026-9030, 1991), Flt-1 (de Vries et al., Science 255:989-991, 1992), and neuropilin-1 (Soker et al., Cell 92:735-745, 1998). Neuropilin-1 is a cell-surface glycoprotein that was initially identified in Xenopus tadpole nervous tissues, then in chicken, mouse, and human. The primary structure of neuropilin-1 is highly conserved among these vertebrate species. Neuropilin-1 has been demonstrated to be a receptor for various members of the semaphorin family including semaphorin III (Kolodkin et al., Cell 90:753-762, 1997), Sema E and Sema IV (Chen et al., Neuron 19:547-559, 1997). A variety of activities have been associated with the binding of neuropilin-1 to its ligands. For example, binding of semaphorin III to neuropilin-1 can induce neuronal growth cone collapse and repulsion of neurites in vitro (Kitsukawa et al., Neuron 19: 995-1005, 1997).
In mice, neuropilin-1 is expressed in the cardiovascular system, nervous system, and limbs at particular developmental stages. Chimeric mice over-expressing neuropilin-1 were found to be embryonic lethal (Kitsukawa et al., Development 121:4309-4318, 1995). The chimeric embryos exhibited several morphological abnormalities, including excess capillaries and blood vessels, dilation of blood vessels, malformed hearts, ectopic sprouting and defasciculation of nerve fibers, and extra digits. All of these abnormalities occurred in the organs in which neuropilin-1 is expressed in normal development. Mice lacking the neuropilin-1 gene have severe cardiovascular abnormalities, including impairment of vascular network formation in the central and peripheral nervous systems (Takashima et al., American Heart Association 1998 Meeting, Abstract # 3178).
Neuropilin-1 has been identified as a cell-surface receptor for VEGF (Soker et al., ibid.), and displays selective binding activity for VEGF165 over VEGF121. It has been shown to be expressed on vascular endothelial cells and tumor cells in vitro. When neuropilin-1 is co-expressed in cells with KDR, neuropilin-1 enhances the binding of VEGF165 to KDR and VEGF165-mediated chemotaxis. Conversely, inhibition of VEGF165, binding to neuropilin-1 inhibits its binding to KDR and its mitogenic activity for endothelial cells (Soker, et al., ibid.). Neuropilin-1 is also a receptor for PlGF-2 (Migdal et al., J. Biol. Chem. 273: 22272-22278, 1998). A second semaphorin receptor, neuropilin-2, exhibits homology with neuropilin-1 but has differs in binding specificity (Chen et al. Neuron 19: 547-559, 1997).
Semaphorins are a large family of molecules which share the defining semaphorin domain of approximately 500 amino acids. This family can be subdivided into multiple subfamilies that contain both secreted and membrane-bound proteins. Select members of these subfamilies, class III (SemD) and class IV (SemD), form homodimers linked by disulfide bridges. In the case of SemD, there is additional proteolytic processing that creates a 65-kDa isoform that lacks the 33-kDa carboxyl-terminal sequence. Dimerization is believed to be important for functional activity (Klostermann et al., J. Biol. Chem. 273:7326-7331, 1998). Collapsin-1, the first identified vertebrate member of the semaphorin family of axon guidance proteins, has also been shown to form covalent dimers, with dimerization necessary for collapse activity (Koppel et al., J. Biol. Chem. 273:15708-15713, 1998). Semaphorin III has been associated in vitro with regulating growth clone collapse and chemorepulsion of neurites. In vivo it has also been shown to be required for correct sensory afferent innervation and other aspects of development, including skeletal and cardiac defects (Fehar et al., Nature 383:525-528, 1996). Other members of the semaphorin family have been shown to be associated with other forms of biology. The human semaphorin E gene is expressed in rheumatoid synovial cells and is thought to play an immunosuppressive role via inhibition of cytokines (Mangasser-Stephan et al., Biochem. Biophys. Res. Comm. 234:153-156, 1997). CD100, a leukocyte semaphorin, promotes B-cell aggregation and differentiation (Hall et al., Proc. Natl. Acad. Sci. USA 93:11780-11785, 1996). CD100 has also been shown to be expressed in many T-cell lymphomas and may be a marker of malignant T-cell neoplasms (Dorfman et al., Am. J. Pathol. 153:255-262, 1998). Semaphorin homologues have also been identified in DNA viruses (Lang, Genomics 51:340-350, 1998) and in poxvirus (Comeau, et al. Immunity 8:473-482, 1998). Transcription of the mouse semaphorin gene, M-semaH, correlates with metastatic ability of mouse tumor cell lines (Christensen et al., Cancer Res. 58:1238-1244, 1998).
The role of growth factors, other regulatory molecules, and their receptors in controlling cellular processes makes them likely candidates and targets for therapeutic intervention. Platelet-derived growth factor, for example, has been disclosed for the treatment of periodontal disease (U.S. Pat. No. 5,124,316), gastrointestinal ulcers (U.S. Pat. No. 5,234,908), and dermal ulcers (Robson et al., Lancet 339:23-25, 1992; Steed et al., J. Vasc. Surg. 21:71-81, 1995). Inhibition of PDGF receptor activity has been shown to reduce intimal hyperplasia in injured baboon arteries (Giese et al., Restenosis Summit VIII, Poster Session #23, 1996; U.S. Pat. No. 5,620,687). Vascular endothelial growth factors (VEGFs) have been shown to promote the growth of blood vessels in ischemic limbs (Isner et al., The Lancet 348:370-374, 1996), and have been proposed for use as wound-healing agents, for treatment of periodontal disease, for promoting endothelialization in vascular graft surgery, and for promoting collateral circulation following myocardial infarction (WIPO Publication No. WO 95/24473; U.S. Pat. No. 5,219,739). VEGFs are also useful for promoting the growth of vascular endothelial cells in culture. A soluble VEGF receptor (soluble flt-1) has been found to block binding of VEGF to cell-surface receptors and to inhibit the growth of vascular tissue in vitro (Biotechnology News 16(17):5-6, 1996).
In view of the proven clinical utility of hormones, there are needs in the art for additional such molecules for use as therapeutic agents, diagnostic agents, and research tools and reagents. These and other needs are addressed by the present invention.